Because of the large global volume of cotton crops grown primarily for fiber production, cottonseed is available in substantial quantities in many parts of the world (Green, et al., 2002). In addition to the fiber production, from the crushing of this seed, around four million tons of cottonseed oil are produced annually, making it the sixth most important plant oil in commerce.
Cottonseed oil is the second most valuable product of cotton behind lint. World cottonseed oil production ranks third behind soybean and canola oil (USDA-FAS, 2013) It's has a distinct 2:1 polyunsaturated to saturated fatty acid composition that makes it an excellent salad and cooking oil with a neutral slightly nutty taste that generally consists of 26% saturated palmitic acid (C16:0), 2% saturated stearic (C18:0), 18% monounsaturated oleic acid (C18:1), and 52% polyunsaturated linoleic acid (C18:2) (www.cottonseed.com).
Cottonseed oil is a valued raw material in the food industry because its high level of the saturated palmitic acid and absence of the unstable linolenic acid (C18:3) impart good stability and flavor properties. However cottonseed oil is often partially hydrogenated to lower the level of polyunsaturates and achieve the very high stability required in deep-frying or the solidity required for margarine hard stock. Thus, partially hydrogenated cottonseed oil contains a relatively high level of nutritionally undesirable saturated and trans fatty acids. Genetic improvement of cottonseed oil fatty acid composition is therefore being sought to avoid the need for hydrogenation and thereby to improve the nutritional value of cottonseed oil products.
Previous studies have focused on manipulating various oil contents for desirable qualities. Transgenic cotton plants have been developed to increase levels of oleic acid (Chapman, et al., 2001). Commercialization of high-laurate canola and high-oleic soybeans has further confirmed the potential of such manipulation in the food and agricultural markets. Liu, et al., 2009, further suggests that the ability to control levels of oleic and stearic acids through RNAi silencing of certain genes opens up the possibility of designer fatty acid compositions through combined and controlled silencing of genes.
Sawan et al. (2001) tested the effects of nitrogen fertilization, plant growth regulators, and zinc on oil properties of cottonseed. All three treatments resulted in a significant decrease in the saturated fatty acids and simultaneously increasing the unsaturated fatty acids.
Green (1986) was able to lower linolenic fatty acid (18:3), using two mutant genotypes of flax (Linum usitatissimum L.) that were treated with EMS. The parental lines contained 19.1 and 23.4% linolenic acid. By crossing the parents he was successfully able to decrease the total C18:3 to 7%. 7% of the F2 progeny had low C18:3 ( 1/16th) which proved the recombination of two unlinked genes. There were four phenotypic classes suggesting additive gene action. When tested in F3 generation the low linolenic lines did not segregate, proving they were homozygous recessive for both mutations.
Li et al. (2002) stated that several mutant lines of soybeans contain reduced levels of palmitic acid had genes for palmitic acid located at different loci in the soybean genome, They were able to locate two genes in soybeans using molecular mapping, that when combined accounted for 51% of the total phenotypic variation for palmitic acid in the F2 population and 43% of the variation in the F2,3 generation.
Miquel et al. (1993) researched the fad2 gene in Arabidopsis and its effects on germination, as well as growth and development. The fad2 gene reduces the amount of polyunsaturated fatty acids in the seed oil, by altering the pathway and creating a higher concentration of the monounsaturated fatty acid, oleic acid. The polyunsaturated fatty acids linolenic and linoleic have a much lower melting temperature than oleic. Miquel and Browse (1994) tested mutant lines of Arabidopsis that contained the fad2-2 gene. Lines were subjected to temperatures of 22° C. (72° F.) then moved to low temperatures of 6° C. (43° F.). Wild-type, and heterozygous lines had excellent germination compared to the homozygous lines that contained the fad2 gene, meaning that the wild-type, containing higher levels of polyunsaturated fatty acids, had a higher tolerance to cool temperatures.
Many studies have further shown the correlation of seed oil content and certain traits, including chilling sensitivity, as cold tolerances if of significant economic value (Nishida, 1996). It is well established that seed oil content affects the membrane lipids, and the level of saturation of such membrane lipids has been correlated to such chilling tolerance.
Additional studies have attempted to show a correlation of percentage fat content within a seed and its cold tolerance, with the intent of obtaining the knowledge of the distribution of the fatty acids in phospholipids of germinating seeds of chilling-sensitive and chilling-resistant plant species may contribute to an understanding of plant susceptibility to chilling injury (Hall, 2003). Adaptation of cotton to low temperatures has been correlated to microsomal omega-6 desaturase (FAD2), as a responsible enzyme for membrane lipid modification (Kargiotidou, 2008). It has further been proposed that plants with lower seed oil melting points and proportions of saturated fatty acids should germinate at cooler temperatures (Meadows, 2012). Linder (2000) even suggests that the proportion of saturated fatty acids in triacylglycerols be suitable as a proxy for melting point, and that such proportions of saturated fatty acids are reduced at higher latitudes. It has therefore been a continued focus to change the proportion of saturated fatty acids to impact the melting point of the overall oil composition.
Because germination temperature is of significant economic interest, the correlations of saturated fatty acid content to melting point have been pursued. However, while indirectly achieved by the increase of oleic acids (see Liu, et al., 2002) which was suggestive of cold tolerance, results have been inconclusive. Meadows (2012) showed inconclusive results in confirming the expectation of germination temperature and the melting point of seed oils. To date, efforts have primarily focused on enhancing the levels of non-saturated oils to further decrease the saturated oils; or, alternatively, to diminish generally the proportions of saturated oil content in the seed. There remains a need in the art for determining optimized levels of oils, including determining which oils are responsible for increased germination at lower temperatures, without affecting the integrity or stability of the seed or resulting plants.
In addition to the economic benefits of the cottonseed oil, cotton producers in northern regions are often faced with very short growing seasons, requiring optimum conditions for increased quality and yields of the cotton, crop. Cotton production in many northern regions is limited by 1) poor stand establishment caused by cool spring temperatures; 2) the lack of heat units in the short growing season; 3) and very cool fall temperatures. Bolek (2010) stated that the ability of cotton to establish a stand of vigorous seedlings under cool temperatures is a key component in the production of cotton in areas experiencing cool temperatures during the early seedling stages. Also due to cotton's indeterminate growth habit an early frost in the fall can be detrimental to cotton fiber and seed quality. Consequently, the ability for a producer to have increased flexibility with planting date would be very beneficial.
While some commercial varieties are more capable of germinating at lower temperatures, there are currently no solutions on the market which provide for cotton lines capable of producing cotton cultivars having seed with low saturated fatty acid content for cottonseed oil production or enhanced germination percentages at significantly lower soil temperatures.